In recent years, lithium secondary batteries have come into remarkably widespread use in information electronics such as notebook personal computers, mobile phones and PDAs (Personal Digital Assistants). For more convenient portable features, the batteries have been rapidly shifted toward size reduction, thickness reduction, weight reduction and higher performance. Further, an attempt is made to employ the lithium secondary batteries for electrical automobiles which are promising as the next generation automobiles. Therefore, the lithium secondary batteries are required to have a higher capacity and a higher output.
The lithium secondary battery includes an electrolyte held between a positive electrode and a negative electrode. The electrolyte is produced by dissolving a lithium salt and additives such as an electrode protection film forming agent in an organic solvent such as propylene carbonate or diethyl carbonate.
In general, LiPF6 and LiBF4 are used as the lithium salt. However, −PF6 and −BF4 serving as counter anions each have a lower anti-oxidation potential, so that a potential range of a positive electrode active substance is not sufficiently utilized. This prevents the automobile batteries from having a higher capacity and a higher output. For example, a higher-performance positive electrode active substance having an anti-oxidation potential of not lower than 7 V (vs Li/Li+) has been development, but cannot be used as the positive electrode active substance material because the counter anion has a lower anti-oxidation potential. For the safety of the batteries in an over-charged state, an electrolyte having an anti-oxidation potential of not lower than 6 V (vs Li/Li+) is demanded, and the anti-oxidation potential is desirably higher by as much as 0.1 V.
Vinylene carbonate is typically used as the electrode protection film forming agent. Vinylene carbonate can be used for formation of a protection film on a negative electrode, but does not have a positive electrode protecting function. Therefore, the potential range of the positive electrode active substance is not sufficiently utilized. Further, it is imperative to ensure the safety of the electrical automobile batteries. In order to eliminate the danger of ignition and explosion due to short circuit, there is a demand for an electrode protective film forming material effective for the positive electrode.
For the prevention of the ignition and the explosion, an attempt is made to replace a part or all of an organic solvent serving as an electrolytic liquid with a flame-retardant and less volatile ionic liquid. However, the ionic liquid has a higher viscosity than the organic solvent. Therefore, an electrolyte prepared by dissolving the lithium salt in the ionic liquid has a problem of a poor electrical conductivity. The term “ionic liquid” herein means a compound which is a salt containing a cation and an anion and having a melting point not higher than about a room temperature.
For the higher performance and the safety of the batteries described above, various lithium salts, electrode protection film forming materials and ionic liquids are proposed. An ion conductive material including a lithium cation and an anion having a specific structure, for example, is proposed as a lithium salt (see, for example, PLT1). Compounds such as 1,3-propanesultone are proposed as electrode protection film forming materials (see, for example, PLT2 to PLT6). Further, an electrolyte containing a specific organic cation, a lithium cation and a nitrogen-containing organic anion is proposed as an ionic liquid (see, for example, PLT7).